1. Field of the Invention
The present invention relates to process control systems. More specifically, the present invention is directed to a method and a process control system utilizing the method in a process controller for minimizing hunting of the process.
2. Description of the Prior Art
In order to perform satisfactory process control using a feedback control system, hunting of a process must be detected at an early stage, and an appropriate countermeasure taken to minimize the hunting. In one conventional system for detecting hunting, when a deviation (e=sp-pv) of a process variable (pv) with respect to a setting parameter (sp) has exceeded a predetermined threshold value e.sub.th, it is thereby determined that hunting has occurred. If the deviation temporarily increases for any reason, this must be prevented from being interpreted as hunting. For this purpose, an oscillation period of a manipulation variable (mv) and a process variable (pv) with respect to a process is calculated, and an interpretation is made based on a change in oscillation period.
In the conventional hunting discriminating system, however, hunting can be detected only after the deviation (or the amplitude of an oscillation waveform) is considerably increased. When the beginning of PID parameter tuning depends only on an operator's request and if process characteristics naturally change, an old PID parameter is left untuned, and optimal control is disabled. In addition, detection timing of hunting is undesirably delayed. In any case, hunting must be detected at an early stage.